Self-supporting thin-film filament detector, process for its manufacture and it applications to gas detection and gas chromatography

ABSTRACT

Detector of the filament type for determining a static or dynamic characteristic of an ambient medium, constituted by a resistive component intended to be heated by the Joule effect in the medium, and an interface process region suitable for reacting with the medium by a physico-chemical with an effect, depending on the characteristic to be determined, on the electrical characteristic of the interface region, in which there is a supporting member through which there is at least one aperture and at least one filament including the resistive component, composed of one or more thin films and a central portion located in the aperture and end portions via which the central portion is connected to the supporting member.

This is a division, of application Ser. No. 08/345,848, filed Nov. 28,1994, U.S. Pat. No. 5,680,418, which is a continuation of Ser. No.07/829,074, filed as PCT/FR90/00608, Aug. 10, 1990, now abandoned.

BACKGROUND OF THE INVENTION

The present invention concerns a filament type sensor for determining astatic or dynamic characteristic of a gas environment such as the air,for example, a method of fabricating it, and applications of the sensorprimarily to the detection of oxidizable gases but also to gaschromatography (detection of ionizable gases) and fluid flowratemeasurement.

A filament type sensor of this kind has a resistive element within afilament adapted to exchange heat with the environment and an interfacearea adapted to react with the environment in a physico-chemicalprocess. In the broadest possible sense of the term, this processincludes catalysis of combustion, adsorption, ionization, simple thermalexchange, as well as others which influence an electrical characteristicof the interface area, i.e., temperature or resistance, voltage,current, etc., according to the characteristic of the environment to bedetermined, for example, concentration, flowrate, etc. The interfacearea can be the external portion of the resistive element, or a catalystfilm heated by conduction, or a separate electrode.

Some sensors of this kind are based on measuring the heat exchanged(detection of combustible gases, flowmeter, etc.) and may becharacterized as calorimetric sensors. There are also various filamenttype sensors having the common feature of measuring a concentration,based on various phenomena; for example, measurement of the heatexchanged in the case of detecting combustible or oxidizable gases, ormeasurement of the quantities of ions captured by an electrode in gaschromatography. Filament type sensors are, therefore, of very diversekinds, both with regard to the physico-chemical phenomenon on whichtheir operation is based and with regard to the nature of the parameterto be measured.

Although the remainder of this description refers for the most part tothe detection of an oxidizable gas in a gas environment such as the air,in the field of explosimetry, for example, this is a preferredapplication and is not limited to the invention.

A known way of detecting an oxidizable gas in the air uses a filament,usually of platinum, heated by the Joule effect, i.e. by the passage ofan electric current. The oxidizable gas contained in the surrounding airis oxidized by catalysis in contact with the filament, so that thelatter is further heated. The resulting temperature variation causes avariation in the resistance of the filament, which is measured directlyor indirectly to obtain the concentration of the oxidizable gas in theair. These filament-based detectors are largely hand-made. They,therefore, suffer from a lack of reproducibility and high cost. Theirlow electrical resistance and their low surface area/volume ratio makeit necessary to operate them at high temperatures, for example, about1000° C.

Other oxidizable gas detectors are based on catalytic beads; they areformed by a metal detector, of platinum, for example, coated withalumina doped with a catalyst, and resemble a small pearl. Thesedetectors age less rapidly, as the associated combustion temperature islower. However, these beads have the disadvantage of significant driftin sensitivity, reduced stability and an increased response time ascompared with filaments.

A third type of oxidizable gas detector is based on semiconductor metaloxides doped with a catalyst. These detectors are formed by a metalheating element which heats an insulative material (alumina, forexample) sleeve onto which is deposited a film of semiconductor materialwhose resistance variations are measured. These detectors are sensitiveto any gas that can be adsorbed onto the surface of the semiconductor.They have a relatively long response time, however, and the furtherdisadvantage of high electrical power consumption; also, the effects ofhumidity are not compensated.

The invention is directed to alleviating the aforementioneddisadvantages by improving reproducibility and by reducing thermallosses from the filament by conduction, while also reducingmanufacturing costs.

SUMMARY OF THE INVENTION

In a very general way, the invention proposes a filament type sensor fordetermining a static or dynamic characteristic of a surroundingenvironment, having a resistive element adapted to be heated in theenvironment by the Joule effect and an interface area adapted to reactwith the environment in a physico-chemical process influencing anelectrical characteristic of the interface area according to thecharacteristic to be determined. The sensor has a supporting waferthrough which there is formed at least one aperture and at least onefilament including the resistive element, composed of one or more thinfilms and having a central portion situated in the aperture and at leasttwo end portions by which the central portion is connected to thesupporting wafer.

In other words, the invention proposes a filament fabricated usingmicroelectronics technology in such a way that it is "self-supporting",meaning that the only connections between it and the support are thinfilms. The filament is, therefore, composed of one or more "floating"thin films, which considerably reduces thermal losses by conduction.

The invention results from the observation that thin film technology canbe used to produce a filament having sufficient mechanical strength andthermal shock resistance for it to be self-supporting.

It has been found that, despite the thinness of the filament whichconfers upon it the necessary electrical resistance, it is bothsufficiently sensitive with respect to the physico-chemical reaction onwhich the measurement is based and sufficiently strong that it is notworn out prematurely through contact with the surrounding environment.

According to preferred features of the invention, the filament is formedby a film of a metal catalyst whose exterior surface constitutes theinterface area. At least the central portion of the filament is formedby at least three superimposed thin films constituting a conductivematerial film extending to the ends of the filament, a catalyst filmforming the interface area and an electrically insulative materialintermediate film. The resistive element of the filament is a film of anoble metal such as platinum, gold or palladium or a combination ofnoble metals. The filament has a sinuous shape, for example, acrenellated shape, and the central portion of the filament is connectedto the substrate by more than two end portions. The substrate is chosenfrom the group of materials including glass, silicon, alumina, silica,quartz and polymers and the interface area is a thin film deposited onat least one surface of the substrate near the aperture.

The invention also proposes a filament preparation method usingmicroelectronics technology suitable for fabricating the aforementionedsensor utilizing the following steps:

depositing onto the front and rear surfaces of a wafer-form substrate athin film front mask incorporating a front window, the shape of whichreflects the shape of the filament to be fabricated and has a centralportion extended by end portions, and a thin film rear maskincorporating a rear window facing the central portion of the windowexcluding the ends, but larger than the central portion;

hollowing a trench into the substrate by etching the front surface ofthe substrate through the front mask;

depositing onto the back of the trench one or more thin films adapted toconstitute the filament, at least one of the thin films being anelectrically conductive material; and

eliminating the substrate to its full thickness by etching it throughthe rear mask.

According to the preferred features of the invention, before thesubstrate is etched through the rear mask to eliminate its entirethickness, a protective film is deposited onto the front surface andinto the trench and the protective film is eliminated after thesubstrate is etched; the protective film on the front surface is apolymer resin; and the front mask is an intermediate film covered with afilm of resin, the thin films of the filament being deposited, afterelimination of the resin film, by deposition of one or more thin filmsinto and around the trench followed by elimination of the thin filmsdeposited outside the trench by etching the intermediate film.

The main advantages of the invention as compared with all the previouslymentioned detector elements are very low electrical power consumptionand a very short response time.

The sensor can be manufactured automatically and in multiples and can,therefore, be fabricated in large quantities at low cost.

The sensor's resistance depends on its geometrical shape and allows anoperating temperature lower than conventional filament type sensors,which results in good measurement resolution and slower aging. It isrelatively insensitive to impact due to its novel construction and itsresulting very low mass.

Because its thermal inertia is very low, it can be used for measurementsat different temperatures and in very short time intervals.

The invention also resides in applications of a sensor of this kind,principally to detecting oxidizable gases such as methane or carbonmonoxide and also to gas chromatography (detection of ionizable gases)and to the calorimetric measurement of gas flowrates.

Other objects, features, and advantages of the invention will emergefrom a reading of the following description taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a sensor in accordance with theinvention;

FIG. 2 is a partial view of another sensor in longitudinal cross-sectionin the direction of its thickness;

FIG. 3 is an alternate embodiment of the sensor depicted in FIG. 1; and

FIGS. 4 through 9 are views of the sensor in a direction along line 4--4of FIG. 1 in cross-section at various stages in its fabrication on asubstrate by the method in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A sensor C depicted in FIG. 1 constitutes a support wafer 1 made fromglass or some other insulative (or semiconductor) material with anaperture 2 through it. As an alternative, the support wafer may be madefrom an insulative or non-insulative material covered with an insulativefilm.

Across the aperture is a filament 3 in the form of a thin film of anelectrically conductive material whose outer surface or skin constitutesan interface area with the surrounding environment.

The filament 3 has a central portion 3A and electrically conductive ends4 by which the central portion 3A is connected to the support wafer 1.The ends 4 terminate at conductive lands 5 to which electrical wires 6connecting the sensor to the remainder of the electric circuit includingit can be connected, by soldering, for example.

The filament 3 preferably has a sinuous shape parallel to the supportwafer 1, in this instance a crenellated shape. For a given cross-sectionand a given distance between the lands 5 this increases its surface areaand reduces the risk of rupture due to thermal expansion. Othergeometrical shapes are possible, of course. The thin film filament 3need not be rectilinear overall, but could be curved parallel to thesupport wafer 1. The filament 3 could equally well be in the form of athin plate parallel to the support wafer 1, with dimensions less thanthose of the aperture 2.

The thin film filament 3 may be produced from any substance giving riseto the physico-chemical phenomena on which the measurements are based;in this instance the thin film filament 3 is made from a material chosento have electrical properties which are modified by the environment tobe characterized.

In the particular instance of oxidizable gas measurement, it may be acatalyst: platinum, nickel, osmium, gold, irridium, combinations ofmetals, metal oxides, semiconductors, sulfides, etc.

The material may also be chosen according to its absorbent or adsorbentproperties if they modify its electrical characteristics.

In FIG. 2 parts similar to parts of FIG. 1 have the same referencenumber with a "prime" suffix. This figure shows another sensor C' whosefilament 3' is not a single thin film but a stack of thin films ofconductive or insulative material or catalyst. The succession of thesefilms is such that each catalyst film is at the top or bottom of thestack; each electrically conductive film is electrically connected tothe lands 5; and an insulative film 8 is provided between the conductivematerial or catalyst films.

To be more precise, FIG. 2 shows three successive films 7, 8 and 9,respectively of conductive material, insulative material, and catalyst.In an alternative embodiment that is not shown the films are stackedwith a 9-8-7-8-9 arrangement.

In FIG. 3 parts similar to parts of FIG. 1 have the same referencenumber with a "double prime" suffix. The figure shows another variant C"of FIG. 1 in which additional end portions 10 are disposed transverselyto the central portion of the filament 3". These additional portions endat lands 10A. In the case of a single-film filament as in FIG. 1, theycan be used for intermediate electrical measurements or in differentcircuits, so reducing the number of different sensors to be manufacturedand stored for a given number of applications. In the case of a multiplefilm filament as in FIG. 2, the end portions 10 may be electricalconnections to the catalyst film which is otherwise insulated from theconductive film.

It will be understood that in each of the foregoing examples all of thefilament is entirely contained within the overall thickness of thesubstrate.

FIGS. 4 through 9 show in cross-section, as viewed in a direction alongline 4--4 in FIG. 1, various stages in the manufacture of the sensor, aglass substrate being used in this example.

A first phase entails preparing the substrate by cleaning it usingnitric or sulfochromic acid, for example, followed by rinsing withdeionized water, and drying under dust-free conditions.

In a second phase, masks are prepared on each of front and rear surfaces1A and 1B of the substrate, in stages, as follows (see FIG. 4).

A thin film 11 of chromium with a thickness of 1000 to 2000 Å isdeposited onto the rear surface; a film 12 of chromium between a few Åand 1000 Å thick and then a film 13 of gold approximately 1000 Å thickare deposited onto the other (front) surface; these stages may bestaggered with respect to each other but are preferably simultaneous; afilm 14 and 15 of photosensitive resin is deposited onto the front andrear surfaces 1A and 1, respectively, of the substrate; exposure masks14A and 15A are positioned on opposite sides of the substrate and facetheir corresponding films 14 and 15, respectively, such that the films14 and 15 are exposed through the masks 14A and 15A and the exposedareas are developed which produces resin masks 16 and 17;

the metal films are etched through the masks 16 and 17, i.e., etching ofthe chromium film 11 on the rear surface, etching of the gold film 13 onthe front surface, and etching of the chromium film 12 on the frontsurface; and further rinsing with deionized water, resulting in thestructure shown in FIG. 5.

It will be understood that the rear mask obtained in this way (films 11and 14) includes a window 16A facing the central portion (between theends 4 and 5 in FIG. 1) of the window 17A in the front mask (films 12,13 and 15), to the exclusion of the ends, but the window 16A is larger(in this instance wider on each side) than the central portion.

In a third phase hydrofluoric acid is used to etch trenches 18 and 19into the glass through the masks consisting of the superimposed films ofchromium 12, gold 13 and resin 15 etched onto the front surface 1A ofthe support wafer 1. This etching is isotropic in the direction of thethickness and laterally; the resulting undercutting leaves the films 12and 13 projecting over inclined edges 20 of the trench to form anoverhang 21. Although it is standard practice with etching methods ofthis type to modify the process conditions to avoid such undercutting,in this instance such undercutting is deliberate and useful. Theresulting overhang 21 allows improved removal of the films 12 and 13 atthe end of fabrication.

The resin masks 14 and 15 are removed, for example, using acetone andthen nitric acid. The resulting structure is then rinsed with deionizedwater and dried under dust-free conditions, yielding the structuredepicted in FIG. 6.

In a fourth stage the filament 3 is formed at the bottom of the trench18 by depositing a thin film 23 of chromium (approximately 100 Å thick)onto the front surface 1A of the substrate, including onto the bottom ofthe trench 18, followed by the deposition of a film 24 of platinum overall of the thin film 23 (see FIG. 7). There are obtained in this waythin films 23A and 24A of chromium and platinum in the trenchdissociated from portions 23B and 24B of chromium and platinum depositedon the remainder of the front surface 1A. The overall thickness of thefilms 23 and 24 must, therefore, be at least slightly less than thedepth of the trench 18. In the case of the sensor C' from FIG. 2 theequivalent condition is that the overall thickness of the depositedfilms must be less than the depth of the trench. It is essential thatthe films in the trench 18 do not come into contact with the overhangs21.

The side portions of the excess platinum and chromium films 24A and 23Aare then eliminated by chemical etching of the gold film 13 (immersionof the substrate for at least three hours in a gold etching reagentwhich mechanically eliminates the superfluous film of platinum, with thefinal traces of excess platinum removed in an ultrasonic cleaning tank).This operation is greatly facilitated by the overhang 21 obtained byundercutting.

Following rinsing with deionized water and drying a new film 25, seeFIG. 8, of photosensitive resin approximately 3 Im thick is depositedonto the rear surface and is then exposed through the same mask 14A asin FIG. 4. Following development, a rear mask is obtained coincidentwith the chromium mask 11 remaining on this surface; in practice themask is then cured at 140° C. for 30 minutes.

In a final phase the substrate is hollowed out through its entirethickness by an etching process through the rear mask, in the followingstages.

A protective film 26 is deposited on the front surface 1A covered withthe chromium film 12; this protective film 26 fills the trench 18 and byadhering to it covers the filament 3 at its bottom; this protective film26 may be of any material which is resistant to hydrofluoric acid andcan be easily dissolved using a commercially available solvent; a ZIVIAPIEZON-W type polymer resin is preferably used; and

the support wafer or glass 1 carrying the filament 3 is chemicallyetched with ultrasonic agitation through the mask 25 deposited onto therear surface 1B and consisting of the chromium film 11 and the etchedphotosensitive resin film 25.

After the glass and the protective film 26 are removed using anappropriate commercially available solvent, such as perchlorethylene,for example, the filament 3 is, surprisingly, found to be"self-supported" in the glass support wafer 1 (see FIG. 9). All tracesof resin and polymer are removed from the glass support wafer 1 using anappropriate chemical reagent (usually fuming nitric acid) and theremaining chromium films 11 and 12 on each side of the glass supportwafer 1 are removed using the reagent for chemical etching of chromium.

The inclined flanks of the aperture 2 in FIG. 9 result from theisotropic nature of the etching by hydrofluoric acid. In the case of asubstrate and an acid producing anisotropic etching, vertical flankswould be obtained as shown in FIGS. 7 and 8.

Specific examples of the chemical etching reagents used are:

chromium:

1) SOPRELEC (EVRY) Cr-ETCH

2) 50 g/l of KMnO4+50 g/l of KOH+1 l of deionized water, gold: 25 g/l ofI2=60 g/l of KI+1 l of deionized water, glass: HF diluted 40% to 20%(according to the required etching rate).

Examples of thicknesses for glass wafers 150 Im thick are:

chromium No 11 : 500 to 1000 Å,

chromium Nos 12 and 23 : 50 to 500 Å,

gold No 13 : 1500 to 2500 Å,

platinum : 0.5 to 9 Im,

W apiezon : 100 Im minimum,

SHIPPLEY 1350-H photosensitive resin : 1 to 3 Im,

length of hole : 2 mm.

The benefits of the chromium films 11 and 12 are firstly the improveddeposition of the gold film 13, which could not be achieved directlyonto the glass, and secondly the high strength of the mask formed by thephotosensitive resin 14 and 15 and chromium films 11 and 12 duringetching of the glass with hydrofluoric acid.

In a variant of the method shown in FIGS. 5b and 6b the gold film 13 and13b is thickened. This makes it possible to deposit a greater thicknessof platinum (as represented by film 24 in FIG. 7).

The second and third phases of the method are modified as follows.

After etching the metal through the masks 16 and 17, the resin layers 14and 15 are cleaned using acetone and nitric acid, and further rinsing isthen carried out using deionized water.

It will be realized that the resulting rear mask (layer 11) constitutesa window 16A facing the central portion (between the ends 4 in FIG. 1)of the window 17A in the front mask (layers 12, 13) excluding the endsbut that this window 16A is wider (in this instance wider on each side)than the central portion.

The gold film 13 is then thickened (FIG. 5b) by electrolyticallydepositing gold (film 13b) followed by rinsing with deionized water.

The thickness of the electrolytic gold plating (film 13b) is determinedby the depth of the trenches to be etched in the next stage and isapproximately 1 Im for a trench depth of 10 to 15 Im. A uniform film 11bof protective photosensitive resin is deposited onto the rear surface.

In the third phase hydrofluoric acid is used to etch the trench 18 intothe glass through the mask consisting of the superimposed films 12 ofchromium and 13 and 13b of gold etched onto the front surface 1A of thesupport wafer 1. This etching is isotropic in the direction of thethickness and laterally; the resulting undercutting leaves the films 12,13 and 13b projecting over the inclined edges 20 of the trench to forman overhang 21. Although it is standard practice with etching methods ofthis type to modify the process conditions to avoid the undercutting, inthis instance such undercutting is deliberate and useful. The resultingoverhang 21 allows improved removal of the films 12 and 13 at the end offabrication.

The resin mask 11b is removed, for example using acetone and then nitricacid. The resulting structure is then rinsed with deionized water anddried under dust-free conditions, yielding the structure of FIG. 6.

Subsequent stages of the process are exactly the same as before.

In addition to glass it is possible to use other substrates, forexample, silicon, alumina, silica and especially quartz which offersgood heat resistance and selective resistance to etching.

It is also possible to use double-sided metal-plated substrates, forexample, gold over chromium, which means that the first metal depositionstages can be omitted.

Trials have been conducted on quartz between 125 and 175 Im thick platedwith gold over chromium on both sides using the same chemical etchantreagents.

There are diverse applications for a sensor of this kind.

First, it can be used to detect oxidizable gas by integrating adescribed known circuit.

Second, it can also be used for chromatographic measurements. Thefilament 3 is used to heat and locally ionize the gaseous medium and oneor more ion receiving electrodes (interface area) are constituted by oneor more conductive thin films deposited onto the substrate near theaperture 2. The chromium films 11 and 12 may be left in place for thispurpose.

It goes without saying that the present invention has been described byway of non-limiting example only and that numerous variants can be putforward by one skilled in the art without departing from the scope ofthe invention. For example, multiple filaments may be formed in a singleaperture and multiple apertures may be formed in a single substrate.

What is claimed is:
 1. A method for making a sensor comprising thefollowing steps:depositing onto a front surface of a wafer substrate athin film front mask having a front opening, said front opening having acentral portion and a plurality of end portions; depositing onto a rearsurface of said wafer substrate a thin film rear mask having a rearopening oppositely disposed said central portion of said front openingsaid rear opening being larger than said central portion but not largeenough to include said plurality of end portions; etching a trench intosaid front surface of said wafer substrate through said thin film frontmask a bottom surface of said trench at least one thin film, wherein atleast one of said at least one thin film is an electrically conductivematerial; and etching said wafer substrate through said thin film rearmask so as to substantially remove said wafer substrate in said regionsof said thin film front and rear masks such that said at least oneelectrically conductive thin film is unsupported at said regions of saidthin film front and rear masks.
 2. A method for making a sensorcomprising the following steps:depositing onto a front surface of awafer substrate a thin film front mask and a layer of an electrolyticmaterial, said thin film front mask and said layer of electrolyticmaterial having a front opening said front opening having a centralportion and a plurality of end portions and depositing onto a rearsurface of said wafer substrate a thin film rear mask having a rearopening oppositely disposed said central portion of said front openingsaid rear opening being larger than said central portion but not largeenough to include said plurality of end portions. etching a trench intosaid front surface of said wafer substrate through said thin film frontmask; depositing onto a bottom surface of said trench at least one thinfilm wherein at least one of said at least one thin film is nelectrically conductive material; and etching said wafer substratethrough said thin film rear mask so as to substantially remove saidwafer substrate in said regions of said thin film front and rear masks,such that said at least one electrically conductive thin film isunsupported at said regions of said thin film front and rear masks.
 3. Amethod for making a sensor comprising the steps of;providing ametal-plated substrate having a front opening on a rear surface, saidfront opening having a central portion and a plurality of end portionsand a rear opening on a rear surface, oppositely disposed said centralportion of said front opening, said rear opening being larger than saidcentral portion but not large enough to include said plurality of endportions; etching a trench into said front surface of said metal-platedsubstrate through said front opening; depositing at least one thin filmonto a bottom surface of said trench, at least one of said at least onethin film being an electrically conductive material; and etching saidmetal-plated substrate through a mask having essentially the same shapeas said rear opening.
 4. A method according to claim 1 furthercomprising the steps of depositing a protective film onto said frontsurface and into said trench prior to said final etching step andremoving said protective film after said final etching step.
 5. A methodaccording to claim 4 wherein said protective film deposited onto saidfront surface is a polymer resin.
 6. A method according to claim 5wherein said thin film front mask comprises an intermediate film coveredwith a film of resin, and wherein said at least one thin film beingdeposited after removal of said resin film by deposition of said atleast one thin film into said trench and around said trench, followed byremoval of said at least one thin film deposited around said trench byetching said intermediate film.
 7. A method according to claim 6 whereinsaid wafer substrate is glass, said thin film front mask comprises agold film covering a chromium film, and said at least one electricallyconductive thin film deposited onto said bottom surface of said trenchis platinum.
 8. A method according to claim 1 wherein a thin film ofmetal catalyst is deposited onto said bottom surface of said trench. 9.A method according to claim 1 wherein at least three thin films aredeposited onto said bottom surface of said trench, said at least threethin films comprising a thin film of an electrically conductivematerial, a thin film of an insulative material and a thin film of acatalyst material.
 10. A method according to claim 1 wherein said wafersubstrate is chosen from the group of materials consisting of glass,silicon, alumina, silica, quartz and polymers.